The present invention relates to a bonded semiconductor wafer having a layered structure alternately stacked with semiconductor layers and insulator layers in two cycles or more and to a novel manufacturing process therefor.
In the technical progress in miniaturization of LSI (Large Scale Integrated circuit) devices in recent years, more perfect isolation, a higher operating speed and higher performance have been pursued and attention has been given to an SOI (Silicon-On-Insulator) wafer as material satisfying such demands.
There has been known a wafer bonding process as one of the manufacturing techniques for an SOI wafer. As a technique utilizing this bonding process, a so-called ion implantation separation process (also called a Smart-Cut (a registered trade mark) process) has been developed, which is described in JP A 93-211128.
This ion implantation separation process is a technique wherein an oxide film is formed on at least one of two silicon wafers, hydrogen gas ions or rare gas ions are implanted into one silicon wafer through an upper surface thereof to form a micro-bubble layer (an enclosed layer) within the interior of the one silicon wafer, thereafter the ion implanted surface is put into close contact with the other wafer with the oxide film interposed therebetween, then heat treatment (separation heat treatment) is applied to the wafers in close contact to separate the one wafer in such a manner that the contact portion is partly left in a state of a thin film at the micro-bubble layer performing as a cleavage plane (a separation plane), and heat treatment (bonding heat treatment) is further applied to the other wafer with the thin film left thereon to reinforce bonding between the surfaces in close contact and thereby manufacture an SOI wafer.
In this process, an SOI wafer is obtained with relative ease in which the separated surface is a good mirror surface and the SOI layer is very high in uniformity. This process is also advantageous in that a raw material can be effectively used since one wafer partly removed by separation can be reused. Furthermore, this process makes it possible that two silicon wafers can be bonded directly with each other without an oxide film interposed therebetween, not limited to the case where two silicon wafers are bonded together, the ion implanted silicon wafer is bonded to an insulator wafer having a different thermal expansion coefficient such as of quartz, silicon carbide and alumina, or by selecting a material other than silicon as a wafer to be ion implanted, a bonded wafer having a thin film of the material can be manufactured.
With the advent of the ion implantation separation process, there is solved a problem of film thickness uniformity which is the greatest problem in a bonded semiconductor wafer, and it has been possible to manufacture a bonded SOI wafer with a film thickness uniformity as excellent as 1 nm or less in film thickness standard deviation across a surface of the SOI layer. This process has made possible application of a bonded SOI wafer to an LSI such as a CMOS as a leading device, which requires an SOI layer in the form of an ultra-thin film and excellent film thickness uniformity, in addition to its conventional applications to a BiCMOS and a power IC.
While an SOI wafer to be used in the above-mentioned applications is of a structure including a single SOI layer formed on a buried oxide film, the applicant of the present application has proposed a technique in a previous application (JP A 99-316154) that the ion implantation separation process is adopted to form a bonded semiconductor wafer alternately stacked with two kinds of layers having different refractive indexes such as a combination of an SOI layer and a buried oxide film in two cycles or more and the bonded silicon wafer is utilized in an optical functional device.
This technique is to cultivate a field where the bonded semiconductor wafer is utilized in an optical functional device such as a waveguide, an optical communication modulator, an optical detector or a laser, and very useful. Especially, an SOI wafer having the simplest two cycle layered structure in such layered structures makes advantageously possible that an upper SOI layer is utilized in fabrication of an LSI while an lower SOI layer is utilized as an optical waveguide or a wiring layer, thereby facilitating three-dimensional wiring.
Description will be given of examples of prior art steps for manufacturing a bonded semiconductor wafer having a two cycle layered structure by use of the ion implantation separation process on the basis of FIGS. 4 and 5. FIG. 4 is a flow chart showing an example of a prior art manufacturing process for a bonded semiconductor wafer. FIG. 5 is a flow chart showing another example of a prior art manufacturing process for a bonded semiconductor wafer.
In FIG. 4, first of all, there are prepared a semiconductor wafer A having an insulator film or an insulator layer 10a on a surface thereof and a semiconductor wafer B having no insulator film on a surface thereof. Hydrogen (or rare gas) ions are implanted into the wafer A to form a micro-bubble layer 12a in the interior of the wafer A. The wafer A with the micro-bubble layer 12a is bonded to the wafer B with the insulator film 10a interposed therebetween at room temperature to form a bonded wafer 15.
When the bonded wafer 15 is heat treated, cracking occurs in the wafer A at the micro-bubble layer 12a due to strain to form an SOI wafer 16 with a one cycle layered structure in which the insulator layer 10a and a semiconductor layer 14a are formed on the wafer B. As opposed to the SOI wafer 16 with a one cycle layered structure, there is further prepared a wafer C having an insulator film or an insulator layer 10b on a surface thereof, hydrogen (or rare gas) ions are implanted into the wafer C to form a micro-bubble layer 12b in the interior of the wafer C. The wafer C with the micro-bubble layer 12b is bonded to the SOI wafer 16 at room temperature to form a bonded wafer 17.
When the bonded wafer 17 is heat treated, cracking occurs in the wafer C at the micro-bubble layer 12b to form an SOI wafer 18 with a two cycle layered structure in which the insulator layer 10b and the semiconductor layer 14b constituting the second cycle layered structure are formed on the first cycle layered structure (the insulator layer 10a and the semiconductor layer 14a) of the SOI wafer 16.
In another example of a prior art process shown in FIG. 5, a manufacturing process goes according to the same steps as in FIG. 4 till the first cycle layered structure is formed on a wafer B and as opposed to the SOI wafer 16 there is further prepared a wafer C having an insulator film or an insulator layer 10b on a surface thereof In this prior art example, hydrogen (or rare gas) ions are implanted into the SOI wafer 16 to form a micro-bubble layer 12c in the interior of the SOI wafer 16 instead of ion implantation on the wafer C in FIG. 4.
Then, the SOI wafer 16 is bonded to the wafer C with the insulator film 10b interposed therebetween at room temperature to form a bonded wafer 17. When the bonded wafer 17 is heat treated, cracking occurs in the wafer C due to strain at the micro-bubble layer 12c, and the insulator layer 10b and a semiconductor layer 14c are further formed in addition to the one cycle layered structure (the insulator layer 10a and the semiconductor layer 14a) of the SOI wafer 16 to complete an SOI wafer 18 having a two cycle layered structure.
As described above, in any of the manufacturing processes of FIGS. 4 and 5, there are necessarily repeated two times the hydrogen (or rare gas) ion implantation step and the bonding step, respectively. Furthermore, at least two heat treatment steps are required for separation heat treatment, and when separation heat treatment and bonding heat treatment are separately performed, further heat treatment steps are added.
In manufacturing the bonded semiconductor wafer having such a layered structure, however, there has to be repeated the bonding step requiring the severest process control for the purpose to improve the manufacture yield, so the manufacturing processes are neither simple nor convenient, resulting in high frequency in occurrence of defective products in the bonding steps.
The inventors have conducted earnest studies on a manufacturing process capable of manufacturing a bonded semiconductor wafer having a cyclically layered structure including semiconductor layers and insulator layers, and especially, a two cycle layered structure which is the simplest and most valuable in utilization among such cyclically layered structures, with more ease and further reduced frequency in occurrence of defective products, and as a result, have reached a conception that the number of bonding steps can be reduced by utilizing an oxide layer generated by oxygen ion implantation as an insulator layer, and completed the present invention on the basis of the conception.
With the above described problems in view, it is an object of the present invention to provide a novel bonded semiconductor wafer having a layered structure alternately stacked with semiconductor layers and insulator layers in two cycles or more and manufactured by means of a bonding process, wherein at least one of the insulator layers is formed with ion implanted oxygen, and a novel manufacturing process for a bonded semiconductor wafer in which an ion implantation separation process is adopted, and capable of simplifying a manufacturing procedure and reducing defective products in the bonding steps.
In order to solve the above described problem, a bonded semiconductor wafer of the present invention is manufactured by means of a bonding process and has a layered structure alternately stacked with semiconductor layers and insulator layers in two cycles or more, wherein at least one of the insulator layers is formed with ion implanted oxygen.
It is preferable that the semiconductor layers are of silicon and the insulator layers are of silicon oxide films, and the number of cycles of the layered structure is 2.
A first aspect of a manufacturing process for a bonded semiconductor wafer of the present invention comprises the steps of: preparing first and second semiconductor wafers on one or both of which an insulator film or insulator films are formed; implanting oxygen ions and hydrogen or rare gas ions into one surface of the first semiconductor wafer; thereafter bonding the surface of the first semiconductor wafer into which the ions have been implanted and a surface of the second semiconductor wafer with the insulator film or films interposed therebetween to form a bonded semiconductor wafer; heat treating the bonded semiconductor wafer, thereby the first semiconductor wafer being separated from the bonded semiconductor wafer at a micro-bubble layer formed by the implantation of hydrogen ions or rare gas ions.
In the first aspect, while an implantation depth of hydrogen ions or rare gas ions is required to be deeper than that of oxygen ions, there is no specific limitation as to which of both of the ion implantation steps is to be first performed.
A second aspect of a manufacturing process for a bonded semiconductor wafer of the present invention comprises the steps of: preparing first and second semiconductor wafers on one or both of which an insulator film or insulator films are formed; implanting oxygen ions into one surface of the first semiconductor wafer; implanting hydrogen ions or rare gas ions into one surface of the second semiconductor wafer; bonding the surface of the first semiconductor wafer and the surface of the second semiconductor wafer into each of which the ions have been implanted with the insulator film or films interposed therebetween to form a bonded semiconductor wafer; and heat treating the bonded semiconductor wafer, thereby the second semiconductor wafer being separated from the bonded semiconductor wafer at a micro-bubble layer formed by the implantation of hydrogen ions or rare gas ions.
A third aspect of a manufacturing process for a bonded semiconductor wafer of the present invention comprises the steps of: preparing first and second semiconductor wafers on one or both of which an insulator film or insulator films are formed; implanting oxygen ions into one surface of the first semiconductor wafer; thereafter heat treating the first semiconductor wafer to make an oxygen ion implanted layer of good film quality; further implanting hydrogen ions or rare gas ions into the one surface of the first semiconductor wafer; thereafter bonding the one surface of the first semiconductor wafer into which the ions have been implanted and a surface of the second semiconductor wafer with the insulator film or films interposed therebetween to form a bonded semiconductor wafer; and heat treating the bonded semiconductor wafer, thereby the first semiconductor wafer being separated from the bonded semiconductor wafer at a micro-bubble layer formed by the implantation of hydrogen ions or rare gas ions.
While the third aspect is a modification of the first aspect, wherein an SOI wafer is once manufactured by means of a so-called SIMOX (Separation by IMplanted OXygen) method and thereafter hydrogen ions are implanted on the SOI wafer, the subsequent steps being performed.